Membrane proteins are divided into two groups based upon the ease with which the proteins can be removed from the membrane. Extrinsic or peripheral membrane proteins can be removed using extremes of ionic strength or pH, the use of urea or other disruptors of protein interactions. Intrinsic or integral membrane proteins are released only when the lipid bilayer of the membrane is dissolved by detergent. Extrinsic membrane proteins comprise the constituents of the cytoskeleton such as spectrin and actin. Many cytoskeletal proteins are bound directly to integral membrane proteins or are bound indirectly via other proteins such as ankyrin. Cytoskeletal proteins control the shape and dynamics of the cell membrane through their interactions with motor proteins such as myosin and dynein.
The majority of known integral membrane proteins are transmembrane proteins which comprise an extracellular, a transmembrane, and an intracellular domain. Transmembrane proteins are typically embedded into the cell membrane by one or more regions comprising 15 to 25 hydrophobic amino acids which are predicted to adopt an .alpha.-helical conformation. Transmembrane proteins are classified as bitopic (or Types I and II) and polytopic (or Types III and IV) Singer, S. J. (1990) Annu. Rev. Cell Biol. 6:247-96!. Bitopic proteins span the membrane once while polytopic proteins contain multiple membrane-spanning segments. A small number of integral membrane proteins, termed monotopic proteins, are partially embedded in the membrane (i.e., they do not span the lipid bilayer). Monotopic proteins may be inserted into the bilayer by a hydrophobic hairpin loop or may be attached to the membrane via bound lipid.
Type II integral membrane proteins have a single transmembrane stretch of hydrophobic residues which is often located near the amino-terminus. The bulk of type II proteins comprises the carboxy-terminal domain which is located on the exterior side of the cell. The amino-terminal domain of type II proteins, located on the cytoplasmic side of the cell membrane, is typically small. Thus, the type II proteins generally lack enzymatically active domains on the cytoplasmic side of the membrane and thus are not themselves directly involved in transmembrane signalling. The carboxy-terminus of type II proteins typically comprises the active portion of the protein (e.g., the active site of an enzyme, the binding domain of a receptor).
Recently a multigene family encoding type II integral membrane proteins, termed the E25 gene family, was identified Deleersnijder W. et al. (1996) J. Biol. Chem. 271:19475!. The best characterized member of this family is the mouse Itm2 gene which encodes the E25AMM protein. The expression of the Itm2 gene was found to be associated with chondro-osteogenic differentiation. The Itm2 gene is strongly, although not exclusively, expressed in osteogenic tissue. In particular, Itm2 is strongly expressed in mature osteoblasts and in early stages of secondary chondrogenesis. Itm2 expression is not limited to chondro-osteogenic tissues as it is expressed in 1) heart, brain (choroid plexus), renal cortex, and the crypts of the small intestine (weak expression) and in 2) skin (stratum corneum), hair follicles and the acini of exocrine glands (strong expression) Deleersnijder W et al., supra!. Additional members of the E25 multigene family have been identified in the mouse and in humans. These additional E25 family members are expressed in a wide variety of tissues including adult and fetal brain, fetal liver, fetal spleen, lung, breast, placenta, prostate, adrenal gland, white blood cells and adult and fetal heart Deleersnijder W et al., supra!.
The discovery of molecules related to the E25 multigene family satisfies a need in the art by providing new diagnostic or therapeutic compositions useful in the treatment of disorders associated with alterations in the expression of members of the E25 multigene family.